Saves the current workitem fields into a variable or into a field.

  save :to_field => 'old_workitem'
    #
    # saves a copy of the fields of the current workitem into itself,
    # in the field named 'old_workitem'

  save :to_variable => '/wix'
    #
    # saves a copy of the current workitem in the varialbe 'wix' at
    # the root of the process

See also the ‘restore’ expression (Ruote::Exp::RestoreExpression).